1. Field of the Invention
The invention relates to a hand-held, electro-optical laser distance meter preferably for use in the construction trade.
2. Description of the Prior Art
A measuring accuracy in the millimeter range is needed in hand-held laser distance meters used in the construction trade for a measurable distance range of up to several hundreds meters. To this end, a bundled, usually visible, laser beam of a laser diode is sent to the surface of an object to-be-measured. The light which is backscattered and/or reflected by the latter is imaged on the active surface of a photoreceiver by receiving optics. The time of flight between transmission of the light and reception of the light is determined by the modulation of the light (pulse-shaped, sine-shaped or stochastic). Modern laser distance meters of this kind work by the pulse reflection mixing method, as it is called. The person skilled in the art is referred to German Publication DE 101 12 833 for details.
These laser distance meters must cover a large usable dynamic range because they must detect very weak measurement light pulses which are backscattered from great distances and from poor targets (e.g., black material) as well as very strong measurement light pulses which are reflected from close range and from reflective surfaces (lacquered surfaces, tile).
In a laser distance meter with a light transmitter offset relative to the receiver axis which is disclosed in German Publication DE 100 51 302, the flat photoreceiver is covered by an opaque shutter in such a way that two separate light-sensitive areas are formed.
In a laser distance meter with a light transmitter offset relative to the reception axis which is disclosed in International Publication WO 030 02 939, a portion of the flat photoreceiver is covered by an opaque shutter. The received light power of the photoreceiver is reduced, and the dynamic range is, therefore, increased because of the displacement of the image measuring light spot caused by the parallax in the near field.
As a rule, the laser diode emits linearly polarized light. In the case of ideal reflective surfaces, the received light is also linearly polarized. In the case of ideal scattering surfaces, unpolarized light is received. Depending on the reflection properties, partially reflecting surfaces produce scattering, unpolarized components as well as reflected, polarized components. In case of surfaces located at great distances, the reflected components, which are also present in seemingly purely scattering surfaces such as, e.g., concrete walls, advantageously contribute to strengthening the measurement signal. Conversely, in the case of surfaces at close range, the reflected components cause extensive interference particularly in highly reflective surfaces because they lead to an overloading of the photoreceiver.